Co-axial piping systems, in which inner pipes are contained within outer pipes, have been used for many years in various applications, such as in heat exchangers, jacketed process piping systems and insulated piping systems. Each of these types of co-axial piping systems have different operating requirements and considerations. In heat exchangers and jacketed process pipes, there is typically fluid flowing through the annulus between the inner and outer piping under laminar flow conditions to maximize the thermal transfer between the piping components and the fluid. In insulated systems, on the other hand, an insulating medium is placed within the annulus between the inner and outer piping components, and thus there is no fluid flow through the annulus.
Typical double-containment piping assemblies include inner piping components mounted within outer piping components, and are intended to transport hazardous or dangerous fluids within the inner piping. If there is a leak in the inner piping, the outer piping is intended to collect any such leakage. The annulus between the inner and outer piping components is normally dry, and it is known to mount leak detection equipment within the annulus to detect the presence of any fluid within the annulus indicating a leak. These types of double-containment piping assemblies have gained popularity only in recent years, which is believed to be due at least in part to legislation enacted in the United States, and similar legislation that has been enacted, or is under consideration in many other countries. In the United States, this legislation includes the Resource Conservation and Recovery Act, which became effective in about 1988, and affects underground piping systems, and the Spill Prevention Control and Countermeasure Rules (SPCC) of the Clean Water Act of 1990, which affect aboveground piping systems.
The requirements of these types of double-containment assemblies are different than the requirements of prior co-axial piping systems, such as jacketed process pipes and double-wall heat exchangers, in several major respects. The annulus in a double-containment piping assembly is intended to be dry, and must be maintained substantially bone dry in many instances in order to avoid false readings by leak detection equipment. If a leak does occur in a double-containment piping assembly, it is necessary to thoroughly flush and then dry the annulus between the inner and outer piping components in order to ensure that the annulus is thoroughly decontaminated.
The flushing and decontamination procedure typically includes the following steps: 1) draining the annulus by safely removing the leaking fluid through low-point drains; 2) flushing a fluid, such as water and/or a neutralizing acid/base, through the annulus to thoroughly decontaminate the annulus and render the system safe for repair and/or maintenance (which may also be required by OSHA rules in the United States); and 3) thoroughly drying the annulus before and after repair of the piping, and before placing the piping back in service. Drying is normally accomplished by introducing dry air (sometimes heated air in order to increase the rate at which moisture is absorbed), nitrogen, or other inert gas, usually at relatively high flow rates through the annulus in order to increase the rate of drying.
During this decontamination process, "fully turbulent flow" (e.g., Reynolds number greater than approximately 10,000) of both the liquid and gas through the annulus is desirable for more effective decontamination. In prior co-axial piping systems, such as conventional jacketed process piping systems and double-wall heat exchanger applications, on the other hand, the systems are designed to have a more laminar flow (e.g., Reynolds number less than 1,000) in order to facilitate heat transfer through the annulus. Typically, interstitial supports, or other structures mounted within an annulus of a co-axial piping system tend to create a frictional resistance to fluid flow. Generally, the greater the obstruction and/or frictional resistance, the more substantial is the decrease in the rate of fluid flow through the annulus, and the more laminar (or less turbulent) is the flow. The shapes of supports in double-containment piping assemblies, such as interstitial supports and internal anchor supports, and the shapes and relative dimensions of apertures formed in such supports for permitting fluid flow through the annulus of a double-containment piping assembly, are therefore critical considerations in controlling the nature of any fluid flow through the annulus. These considerations have generally gone unrecognized in double-containment piping assemblies to date.
With collar-type interstitial supports, a substantial portion of the inner and outer peripheries of the supports are maintained in contact with the walls of the inner and outer piping. As a result, with collar-type supports there is a relatively even load distribution between the inner and outer piping. With vane-type supports, on the other hand, there is a ring portion that surrounds the inner piping, or is coupled between sections of inner piping, and a plurality of relatively narrow vanes projecting outward from the ring portion and contacting the outer piping for supporting the inner piping within the outer piping. Because the vanes are relatively narrow in width, the load created by the inner piping resting on the inner wall of the outer piping is concentrated into point loads where the relatively narrow vanes contact the outer piping. The load distribution is therefore not as uniform or even as with collar-type interstitial supports. This is particularly the case when there are relatively few narrow vanes, e.g., five or less vanes, each vane occupying less than an approximately 5 degree section of the annular surface of the fitting, or having a thickness of schedule 80 piping or less.
This type of load (and stress) concentration is amplified when there are movements of the primary piping relative to the containment piping due, for example, to differential thermal expansion or contraction, vibrations, or when there are soil loads on the containment piping, or vehicular traffic loads on the soil above the containment piping, which force the containment piping against the vanes of the support. These types of loads, which can be cyclical, can erode or gouge the areas of contact of the vanes against the containment piping, which can in turn degrade the structural integrity of the containment piping, and lead to failure. A failure of this type in underground installations can cause contamination of ground water supplies.
These difficulties arising from stress concentration become more acute when notch-sensitive materials are employed to form the outer piping, e.g., high-density polyethylene, polypropylene, polyvinylidene fluoride, glass-reinforced epoxy and glass-reinforced vinyl ester, each of which are commonly used to form outer piping in double-containment assemblies. These same difficulties occur when the outer piping is formed from certain frangible materials, and non-ductile materials at low temperatures.
Although these drawbacks associated with stress concentration are typically not encountered with collar-type supports, the collar-type supports known to date define circular, semi-circular, or tangential (or chord) cutouts in the outer peripheries of the fittings for permitting fluid flow through the annulus between the inner and outer piping. These cutouts typically define a relatively small area within the annulus, and therefore tend to substantially decrease the fluid flow rate through the annulus, and lessen the turbulence of the flow. These types of fittings are therefore not best suited for facilitating flushing of the annulus for decontamination following leakage.
Collar-type supports have not been used in most conventional jacketed process piping systems and double-walled heat exchangers, typically because the piping is made of carbon steel or stainless steel, and the systems operate at temperatures in which these materials are ductile. Vane-type supports have normally been preferred in these types of systems because laminar flow enhances thermal transfer, and stress concentration is not a serious consideration.
If a vane-type support were employed in a double-containment assembly, however, not only would there be difficulties associated with stress concentration, as described above, but "vortice shedding" can occur. When fluid flows through a vane-type support, a vortex is typically formed on the downstream side of the support, and a hollow area (i.e., no fluid flow) is thus formed on the downstream side of the support. If a critical flow is achieved (depending upon the flow rate and vibrational characteristics of the piping assembly), the vortices induce vibrations in the piping assembly. If the natural harmonic frequency of the piping assembly is achieved, catastrophic failure can occur.
Although collar-type supports typically avoid vortice shedding, and more uniformly distribute the loads transmitted between the inner and outer piping in comparison to vane-type supports, the annular cutout geometry provided to date in collar-type supports does not enhance, and may prevent fully-turbulent flow in the annulus for cleaning and drying operations necessary for proper decontamination.